1. Field of the Invention
The present invention relates generally to circuits for measuring electrical capacitance and in particular to a circuit for comparing the relative value capacitances, the circuit finding use in precision electronic sensors and the like.
2. Background of the Invention
A wide range of sensor applications use capacitors as sensing elements. The signal to be measured modifies the capacitor and a sense circuit detects changes in the capacitance of the capacitor to produce a corresponding electrical output.
Position, for example, can be measured by attaching opposite plates of a capacitor to separate structures which will be moving with respect to each other. The capacitor formed by the plates will have a capacitance that is a function of the plate""s separation. Thus measurement of the capacitance can provide an electrical output indicating relative position of the structures.
Capacitance position sensors are an attractive option for Micro Electro-Mechanical Systems (MEMS). Microscopic movable plate capacitors are easily integrated into MEMS and fabricated using the same integrated circuit techniques used to construct the MEMS. Unfortunately, at the MEMS scale, the changes in capacitance that can be effected by typical movements of the MEMS components are very small, for example 10xe2x88x9217 Farads. What is needed is a precise and stable circuit capable of resolving such small capacitance changes.
The present invention provides a simple and accurate measurement of the relative size of two capacitors by connecting them in series, then monitoring the voltage at their junction as the series connected capacitors are alternately connected in a first polarity and a second polarity across an arbitrary power and ground connection. If the capacitors are exactly equal, the voltage at their junction will not change. If they differ, the voltage at the junction will be greater when the smaller capacitor is connected to ground and the larger capacitor is connected to the power connection. The changes in voltage, as a function of the switched polarity, may be detected with a comparator storing an earlier junction voltage in a reference capacitor for comparison at the next polarity switch with a later junction voltage. The offset voltage of the comparator may be nulled by modifying the voltage stored in the reference capacitor by the offset voltage provided by the comparator itself during a calibration mode.
Importantly, switching noise from the switching of the polarity of the series connected capacitors is managed by disconnecting the sensing comparator during the switching period.
Specifically, then, the present invention provides a capacitance comparison circuit for comparing the capacitance of a first and second capacitor. The circuit includes a switching network connecting the first and second capacitors to connect them in one of two modes. In the first mode, the first and second capacitor are in series between the power and ground connections with one terminal of the first capacitor connected to power and one terminal of the second capacitor connected to ground. In the second mode, the first and second capacitor are in series between the power and ground connections with one terminal of the second capacitor connected to power and one terminal of the first capacitor connected to ground. A voltage monitor compares the voltage at a junction of the first and second capacitor in the first mode to the voltage at the junction between the first and second capacitor in the second mode to provide an output signal indicating which of the first and second capacitors has greater capacitance and the switching network disconnects the voltage monitor in between the first and second modes.
Thus it is one object of the invention to provide a simple means of precisely comparing two capacitances without the need for precision voltage, time or capacitive references. The power connection may have an arbitrary voltage, so long as it is stable between the first and second mode, and the timing of the first and second modes is flexible. Disconnecting the voltage monitor from the switching capacitors during the switching process allows improved resolution of capacitor values to be obtained. In this way, a slight asynchrony between the operations of the switches of he switching networks such as might otherwise produce errors in the measurement, is tolerated.
The voltage monitor may include a third capacitor storing the voltage at the junction of the first and second capacitor in the first mode for later comparison to the voltage at the junction of the first and second capacitor in the second mode.
Thus it is another object of the invention to provide a simple means of comparing a voltage at two different times.
The voltage monitor may impose a reference voltage on the junction between the first and second capacitances during the first mode and the reference voltage may be substantially half a voltage of the power connection.
Thus it is another object of the invention to provide a well characterized voltage on the capacitors that will be within the operating range of circuitry used to analyze the change in voltage, such circuitry which will share the ground and power connections.
The voltage stored on the third capacitor may be a difference between the voltage at the junction of the first and second capacitor in the first mode and an offset voltage of a differential amplifier used to later compare the voltage on the third capacitor to the voltage at the junction of the first and second capacitors in the second mode.
Thus it is another object of the invention to null the effects amplifier offset voltages on the evaluation of the first and second capacitor such as allows differences in capacitance between these two capacitors to be more finely resolved.
The foregoing objects and advantages may not apply to all embodiments of the inventions and are not intended to define the scope of the invention, for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.